-Choose any number (by chance or conscious decision). Let’s go with 3.
-Use that number to make compositional decisions. Examples:
-number of pitch classes to be used [C, E, F]
-number of dynamic levels allowed [pp, mp, ff]
-number of movements 
-phrase length [as near to 3 measures as the rhythmic content allows]
-particular rhythms allowed (e.g., use the number to decide the constituents of motives within a set number of base rhythmic units) [will vary by the 3 basic units selected; rests and subdivisions may be used]
-number of instruments/voices to write for [3 violas]
You can see how something as simple as picking one number could set in motion an entire piece of music! You could use this method as is, or as a scaffolding on which to construct other ideas.